The problem of how best to use self-assembly (SA) to organize meso-scale components into three-dimensional (3D) assemblies is unsolved: such structures tend to be dominated by gravitational forces, rather than by interaction between the components. In fact, there are no general strategies for assembly in 3D, other than those involving mechanical processes, such as the use of machines directed or programmed by humans. Self-assembly in 3D at the molecular scale is, of course, ubiquitous, but these processes operate under different constraints than those for larger objects: the molecular forces experienced by thermal collisions—Brownian motion—are larger than forces due to gravity, and molecules remain suspended in solution indefinitely.
In the laboratory, processes based on self-assembly, are most successful at surfaces (e.g., in two dimensions). Examples at different scales include the assembly of ordered monolayers of alkanethiolates (SAMs) on gold, of colloid particles into crystals and photonic band-gap structures of bubbles into crystalline bubble rafts, of microspheres into ordered arrays and of chips onto credit cards. The presence of a templating surface both simplifies and limits self-assembly. In general, the most successful laboratory demonstrations of self-assembly use a single kind of a simple component (e.g., uniform spheres), such as crystals of spheres that have been explored actively for use in photonics, optics, and electronics. There are, of course many elegant examples of self-assembled molecules and molecular aggregates (for example, molecular crystals, liquid crystals, phase-separated block copolymers, proteins, and protein segregates). The processes that generate these structures are, however, not easily subject to design or adapted to non-molecular components, and the most “elementary” of them (e.g., crystallization of simple organic molecules from non-polar solvents) still seem intractably difficult to model or simulate.